This invention concerns a method for deep-frying products to be deep-fried and a device for carrying out this method.
In conventionally designed immersion fryers a relatively few products to be deep-fried are fried in a relatively large amount of hot fat.
The quality features of products that have been deep-fried are a fine crust, a soft hot inside and a low fat content which, however, must not be so low as to reduce the typical fried taste.
In traditional immersion fryers, the exceptionally low thermal conductivity of the fat requires a large amount of fat to act as a heat accumulator so that, once the products to be deep-fried are placed in the fat, the drop in temperature in the frying zone is limited to such an extent that the crust formation prevents the products to be deep-fried from being fully soaked with fat as quickly as possible. The large amount of fat in the immersion fryer must be heated all day long because of the long heating-up time. What is more, around the heating elements temperatures of up to 300xc2x0 C. predominate, which means that the thermal gradient is 130xc2x0 C. at a target fat temperature of 170xc2x0 C. This partial overheating and the thermal permanent load of the fat contribute to a considerable extent to a reduction in the quality of the fat.
In the system of a conventionally designed immersion fryer (standing liquid, heating by means of an enclosed heating coil or via the external walls) during deep-frying the drop in temperature in the deep-frying zone is intensified when the quantity of fat is reduced, since the cooling effect resulting from the introduction of products to be deep-fried is not offset by additional heating owing to the extremely slow thermal convection in the medium. An example of this is standard household fryers, which always produce deep-fried products of inferior quality.
That is why, in system gastronomy, the method used is to keep on increasing the quantity of fat and hence the storage of heat in the immersion fryer, resulting in a products to be deep-fried-to-fat ratio of currently up to 1:29, which corresponds to two baskets containing 680 grams of chips in 40 liters of fat; this ratio may even go up to 1:58, since only one basket is often used. But here too, because of the slow convection of heat from the heating surfaces to the deep-frying zone, the added effect is limited and the deep-frying quality can be improved further.
On the other hand, reducing the amount of fat is desirable since this has a number of advantages during deep-frying, chips (referred to in the U.S. as french fries, but referred, in this application by the British term xe2x80x9cchipsxe2x80x9d) absorb approximately 5% of their original weight in deep-frying fat. This fat is taken out of the system and must be replaced by fresh fat in order to maintain the overall quantity of fat. In conventionally designed immersion fryers this does in fact improve the quality of the fat for a short time, but the harmful substances that are released accumulate over time, with the result that the quality of the fat continues to fall until the limiting values are exceeded after about 20 to 30 hours of operation and the entire contents of the fryer have to be discarded. The most important harmful substances are total polar materials and polymers. They arise out of the degraded products of the products that have been deep-fried, and out of chemically transformed fat components (see above: thermal permanent load and partial overheating). After approximately 4 to 5 operating hours, the quality of the products that have been deep-fried diminishes with the rise in the amount of harmful substances that are released, hence apace with the progressive worsening of the quality of the fat, since the harmful substances are absorbed by the products to be deep-fried. The refreshing effect brought about by replacing the fat that has been removed does not last and cannot ultimately prevent the quality of the fat from falling to the legally prescribed point at which the fat must be renewed (24% for total polar materials, 12% for polymer components). By reducing the amount of fat in proportion to the amount of products to be deep-fried, the refreshing effect brought about by replacing the fat that has been absorbed increases proportionally (since the quantity of fat that has been removed and needs to be replaced remains constant in relation to the quantity of the products to be deep-fried, while the amount of fat that needs to be refreshed is reduced) until finally the harmful substances no longer accumulate beyond the limiting values, but settle around a constant value which gets smaller as the products to be deep-fried-to-fat ratio gets lower.
With such an effective refreshing method there is no longer any old fat that needs to be discarded and so a quality of fat can be achieved that is consistently good, but, using the state-of-the-art immersion frying method, adequate convection of heat to the frying zone to produce quality frying cannot be achieved. Because the temperature is too low and cannot rise, so much fat penetrates the products to be deep-fried before the crust is formed that the quality of the products to be deep-fried drops massively (in this case not through the accumulation of harmful substances but as a result of not achieving the aforementioned quality features).
Methods and devices for deep-frying products to be deep-fried in hot fat are also known, in which the fat is pumped into a cycle in which it is heated up, brought into contact with the products to be deep-fried and filtered. In this case, a very large quantity of hot fat is always brought into contact with a relatively small quantity of goods to be deep-fried during the deep-frying process. The disadvantage of this is that, for example, the large amount of fat cannot be heated up quickly for individual frying processes. As a result, the fat must be kept at a constant operating temperature of between 170xc2x0 C. and 190xc2x0 C. and it is similarly exposed to a damaging permanent load. Nor can a refreshing effect like the one described above be achieved. What is more, the state-of-the-art method is very cost intensive and also harmful to the environment, since a very large quantity of fresh fat is always needed to deep-fry a relatively small amount of products to be deep-fried. The advantages of this cycle frying method over the immersion frying method relate only to the temperature distribution in the frying zone.
According to the state of the art it is therefore impossible to produce deep-fried products of a qualitatively high standard by capitalizing on the advantages of a reduced quantity of fat.
Proceeding on the basis of the existing state of the art, this invention is based on the problem of coming up with a method for deep-frying products to be deep-fried and a device for carrying out this method which, by minimising the quantity of fat used, guarantees the consistent quality of the fat, avoids the emergence of old fat that has to be disposed of and limits the use of fresh fat for the replacement of fat that is discharged together with the products that have been deep-fried. At the same time, this method generates sufficiently high temperatures in the deep-frying zone, which can be maintained during the deep-frying process, in order to achieve the quality of deep-fried products that meets the requirements of collective and individual gastronomy, to improve this quality even further and to maintain it at a constantly high level for an unlimited period of time. In addition, the thermal permanent load and partial overheating of the fat must be avoided so as to limit the build-up of harmful substances.
By keeping the quantity of fat circulating in a circuit or cycle as small as possible, the quantity of fat circulating in the fat cycle is always sufficient not to break the cycle. The small amount of circulating fat allows extremely short heating-up times, with the result that the fat can be heated up for each deep-frying process according to requirements. Constantly keeping a large quantity of fat hot, which would lead to a damaging permanent load of the fat and would also be very energy intensive, is thus avoided. Since a relatively large proportion of the small total amount of fat in the cycle is discharged together with the products that have been deep-fried, using the method according to the invention, the fat that is discharged together with the products that have been deep-fried is always being replaced by fresh fat, since fat is constantly being fed through, with the result that the circulating quantity of fat can be kept at a consistently low level. In this way, the quantity of fat is kept to a minimum and the pumping cycle is not broken as a result of a lack of fat.
Especially when compared with state-of-the-art immersion fryers or cycle fryers using a traditionally large quantity of fat, the deep-frying method and corresponding device according to the invention provide at least the same quality of the products that have been deep-fried with a much lower use of fat and produces a much improved constant fat quality. If, for example, the usual 680 grams of chips are fried in 2.5 liters of fat (ratio of approx. 1:3.7) instead of in 10-20 liters, as in the case of immersion fryers, then, with a fat absorption of 5% of the original weight of the deep-fried products, the entire quantity of fat can be completely replaced by fresh fat after around 74 fryings. This refreshing effect outpaces the contamination of the fat in the small total quantity of fat and means that the quality of the entire quantity of fat remains good and any replacement of fat is eliminated. As a result of the small total quantity of fat, short heating-up times can be attained in comparison to the traditional immersion frying method, which means that permanent heating is not necessary. By saving fat and energy, operating costs can be cut by at least DM 1,400 a year when compared to a comparable 101 immersion fryer.
Because of the way in which heating takes place over large heating surfaces in a thinner layer and with a quicker flow, this system means that, for the first time, the cooling effect caused by the products to be deep-fried is offset in a small total quantity of fat to such an extent that a constantly high deep-frying temperature results. In addition, the shortest deep-frying times and the lowest saturation of the products with fat are possible because the heat is directed to the surfaces of the products to be deep-fried by the fat that is flowing through with an intensity that was not achieved before. This type of heating also gives rise to considerably reduced thermal gradients, with the result that partial overheating on the heating surfaces is avoided.
An advantageous development of the method according to the invention provides that the ratio of the total weight of the products to be deep-fried to the circulating fat lies between 1:1.5 and 1:6, preferably between 1:2 and 1:3.5. This makes it possible for the fresh fat that continually replaces the fat that has been discharged together with the products that have been deep-fried to account for such a large proportion of the reduced total quantity of fat that this is always of good quality and the emergence of old fat that needs to be disposed of is completely avoided.
Another advantageous embodiment of the method according to the invention provides that fat flows through a vat to accommodate the products to be deep-fried, which is located in the fat cycle, within 4 to 6 seconds. This rapid circulation of the small quantity of fat enables the surfaces of the products to be deep-fried to be heated up extremely quickly, which is not possible with any other existing method. The fat that has cooled down through being in contact with the products to be deep-fried is quickly carried away and replaced by flowing hot fat. The ensuing rapid xe2x80x98sealingxe2x80x99 of the products to be deep-fried causes low fat absorption and ensures that the products to be deep-fried are deep-fried in a very short time.
An advantageous development of the device according to the invention provides that the feed direction of the pump can be reversed and that the pump has a reversing device with a time switch and which, after a predetermined frying time, reverses the feed direction of the pump. This means that, at the end of a pre-specified frying time, the feed direction can be reversed, for example by toggling the poles of the pump, until the remaining fat is sucked out of the vat together with the deep-fried products. In this way, the basket lift device, which is becoming increasingly common with currently designed heavy duty fryers and incurs manufacturing costs of approximately DM 1,000 is replaced by a switch that costs less than DM 100. What is more, this provides the fast-food industry in particular, as one of the main users of deep-frying technology, with a device that makes the quality of the deep-fried products largely independent of the attentiveness of badly trained assistants. Negligent or malicious frying in putrid fat is ruled out with this invention. Frying can only take place if there is enough fat in the cycle. Fat shortfalls are always replaced by the necessary fresh fat intake, which means that the deep-frying fat in the cycle is always in good condition. The higher manufacturing costs of devices according to the invention as compared to heavy-duty fryers without basket lift devices are recouped in less than a year. Furthermore, in comparison to basket lift fryers, devices according to the invention are unrivalled in both price and performance.
Another advantageous development of the device provides that the space between the bottom of the vat and the products to be deep-fried is limited on the side of the products to be deep-fried by means of a swage block whose holes are arranged and/or dimensioned in such a way that fat flows out of the space into the products to be deep-fried evenly distributed over the surface of the swage block. This enables all the products to be deep-fried to come into uniform contact with the hot fat.
Another advantageous embodiment of the device provides that a basket to accommodate the products to be deep-fried can be placed inside the vat and that the upper edge of the basket extends out over the upper edge of the vat, which is open at the top, and that, at least in the area extending out over the upper edge of the vat, the basket has holes which retain the particles that have separated from the deep-fried products in the basket. This means that particles that have separated from the deep-fried products do not get into the fat cycle and hence impair the quality of the fat, since the particles can remain in the basket together with the deep-fried products and can be taken out of the cycle together with the deep-fried products.
Another advantageous development of the device provides that, in the side wall of the vat for accommodating the products to be deep-fried, close to the bottom of the vat, there is an opening to feed the fat back to the heating device, which is open whilst the fat is being heated and closed whilst frying is taking place. This enables the fat in the cycle to be heated up very quickly.
Another measure according to the invention for quickly heating up the fat to the correct temperature for deep-frying is to place the products to be deep-fried inside the deep-frying basket before the deep-fryer is switched on, without paying heed to the current temperature of the fat. This additional, especially advantageous, development of the method and the device provides that, once the deep-fryer is switched on, the pump initially pumps the deep-frying fat into a small cycle by directing the fat over a temperature-controlled magnetic valve or thermostatic valve directly into the upper area of the fat collection zone. Once the fat has reached the correct temperature for deep-frying (e.g. 180xc2x0 C.), the valve closes and the hot fat is fed into the products to be deep-fried via one of the pipes passing in front of the valve. This sets off the deep-frying cycle, which has already been described. The end of the deep-frying process is indicated by means of an acoustic or optical signal after a period of time that has been determined empirically. In this way, the operators are not required to keep an eye on the temperature of the fat.
Another advantageous development of the device provides that the vat for accommodating the products to be deep-fried consists of a basic part, which is not very tall, and a leak proof part that can be inserted inside the basic part and is tall enough to accommodate the products to be deep-fried. Whilst the fat is heating up, the insertable part is removed and then replaced for the frying stage. This means that, with the insertable part removed during the heating-up phase, the fat can be heated up very quickly (analogous to the opening of the opening for the feedback in the embodiment described above).
An embodiment of the invention is a removable deep-frying vat, over the upper edge of which the deep-frying fat overflows. The deep-frying vat is connected to a filtering bottom, which covers the bottom of the outer receptacle. The filtering material of the filtering bottom is interrupted under the edge of the deep-frying vat, all the way around, such that there are two filtering surfaces which are mechanically separated from each otherxe2x80x94a central surface, as the bottom of the deep-frying vat, and a surface encircling the central surfacexe2x80x94which together form the bottom of the filter. The deep-frying vat and the filtering bottom can be removed and are locked in place and sealed as shown in FIG. 11 (basket with integrated filter). A standard deep-frying basket made from wire netting is placed inside the deep-frying vat. The fat rises in the deep-frying vat, flows through the products to be deep-fried in the basket and runs over the upper edge of the deep-frying vat to run off through the encircling filtering bottom. As an alternative to the encircling part of the filtering bottom, separately removable filter bags are inserted circumferentially between the outer receptacle and the deep-frying vat, which sit tightly on the edge of the deep-frying vat, with the result that overflowing fat reliably flows away through the filter bags.
An especially attractive embodiment of the invention is a basket with side walls that are closed, apart from the overflow holes. The height at which the overflow holes, which are equipped with the appropriate filtering material, are located determines the extent to which the basket can be filled with the products to be deep-fried. The closed side walls replace the already described deep-frying vat into which a wire basket is inserted. In the laterally closed deep-frying basket the products to be deep-fried lie on a wide-meshed wire bottom. Underneath this wire bottom, on three sides, there is an slide-in cleat for a replaceable filtering bottom. On the remaining open side the filtering bottom can be removed horizontally. Alternatively, the filtering bottom can be locked below or in the basket with a state-of-the-art clamping mechanism. Using another locking device, the basket is held tightly on the bottom of the outer receptacle so as to prevent rising fat from getting between the basket and the outer receptacle and flowing away. FIG. 11 shows a possible form of the locking device using spring steel elements. Sealing is guaranteed by means of a sealing band, made out of PTFE for example, which is attached circumferentially under the edge of the basket in the bottom of the outer receptacle.
Another advantageous embodiment of the device according to the invention provides that a pipe to feed the fat into the vat to accommodate the products to be deep-fried ends inside the confines of the vat and at the height of its upper edge and that there are outlets at this end. This enables the advantages described at the outset to be achieved with a device that directs the fed-in hot fat onto the products to be deep-fried from above.
Another advantageous embodiment of the device according to the invention provides for an outlet cock on the deepest side of the cycle at least to allow the fat to drain away. This means that, for cleaning purposes, the fat can be removed completely from the device and the device can be flushed with cleaning fluid in order to meet hygiene requirements.
Other advantageous embodiments of the device according to the invention and the method according to the invention are given in the remaining subordinate claims. This invention will now be illustrated with the help of figures.